Compensation of Nonlinear Distortion and Group Delay in OFDM Systems

• Main Authors: Meng-Che Liu, 劉孟哲
• Other Authors: Mao-Ching Chiu
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/34140765612291252547

碩士 === 國立中正大學 === 通訊工程研究所 === 95 === OFDM signal is very sensitive to the nonlinear distortion mainly caused by a power amplifier (PA) as a result of its high peak-to-average power ratio (PAPR). Predistortion, an effective countermeasure for balancing off the nonlinearity of a PA, is usually necessary for the sake of mitigating in-band distortion and spectrum regrowth. In the conventional schemes, a memoryless polynomial is generally used in modelling the PA characteristic or designing the predistorter. The polynomial coefficients are solved by least square (LS) estimation or adaptive identification algorithms in the time domain. However, most of the time domain schemes are not easy to implement owing to the practical difficulty in compensating the delay effect caused by the transmission filter and the receiving filter in the feedback path. In this thesis we examine this issue from frequency domain. The PA characteristic (or its inverse) can solved by least square (LS) estimation. But adaptive algorithm seem the only way to approach optimum predistorter. Due to we don’t wish to estimating the characteristic of the PA, So we can use numerical methods to solved this problem. In this thesis we propose adaptive finite difference algorithm to design predistorter, and time delay effect practically rotates the phase on frequency domain. If input signal of the PA is symmetric in frequency domain, out signal of the PA still exhibits the frequency symmetry. When delay effect occur, the phase didn’t still symmetry, thus we can use this characteristic obtain phase difference on differ frequency, consequently we can kwon delay time. Finally, the simulation of application of the proposed schemes to a 64-QAM OFDM system is presented.